Auxiliary DC Fault Clearing Strategy for Low Capacitance Half-bridge MMC

Abstract

By harmonic current injection to reduce the capacitance of bridge arm sub modules (SMs), the volume and weight of MMC can be effectively reduced, and the lightweight of MMC can be realized. Based on the theoretical analysis of the characteristics of low capacitance MMC in DC fault condition, the problem of fast discharge of SMs exists under low capacitance when other parameters remain unchanged, which may lead to low voltage blocking of SMs during the clearing process of DC fault. From the perspective of equivalent charge, a DC fault clearing strategy based on low capacitance half-bridge MMC is proposed. By adjusting the total number of SMs in each phase according to the capacitance reduction ratio in MMC during the fault clearing process, the discharge speed of the SMs’ capacitor can be effectively reduced and the recovery of the system can be accelerated. Based on PSCAD / EMTDC environment, a simulation program is built to verify the effectiveness of the auxiliary strategy of DC fault clearing in low capacitance MMC.